1. Field of Invention
The present invention relates to membrane filtration systems and, more particularly, to submerged membrane filtration systems and their operation.
2. Discussion of Related Art
Submerged membrane filtration process with air scrubbing emerged in the 1980's. The driving force for filtration by suction or static head instead of pressurization was the elimination of the need for a pressure vessel to contain membrane modules, resulting in significant savings on the capital expense of a membrane filtration system. The gas/air consumption, required to scrub the membranes, however, was found to be a dominant portion in operating energy used in such a filtration process which resulted in high than expected operating costs. Consequently, efforts have been undertaken to reduce the gas/air consumption since the introduction of such systems.
There have been two main directions followed: a) improving the membranes' property with low fouling rate and high permeability; and b) improving the filtration/cleaning process.
Several factors can influence the scrubbing efficacy of a membrane to improve the cleaning process. Air could be more efficiently used by re-arranging modules to have a smaller footprint. In this way the amount of air could be concentrated to more efficiently scour the membranes. The use of high packing density modules also saves air consumption per membrane area. Intermittently scouring membranes with air instead of continuous injection is another way to save air consumption.
Another technique uses a mixture of gas and liquid to scrub the membrane. This method is may be advantageous in membrane bioreactor applications where the membrane filters mixed liquor containing a high concentration of suspended solids and a recirculation of mixed liquor is required to achieve denitrification. This method exploits a mixed liquor recirculation flow to scrub the membranes with air, to minimize the solid concentration polarization near the membrane surface and to prevent the dehydration of mixed liquor. The design of the membrane module aims to achieve a uniform distribution of the two-phase mixture into the membrane bundles. Membranes in known modules are typically either freely exposed to the feed or restricted in a perforated cage. Therefore there is still a certain loss of energy during the fluid transfer along the modules.
In the early stage of membrane process development, cross-flow filtration was commonly used, where a shear force was created by pumping a high velocity of feed across the membrane surface. Because more energy is required to create a high shear force to effectively clean the membrane, the application of the cross-flow filtration process is now limited, mainly to tubular membrane filtration applications.